Hydraulic overload safety device



w. scHLA'r'rER r-:rAL ,3,190,141 HYDRAULIC ovEnLoAn SAFETYvDEvIcE June 22, 1965 f f UNI..-

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2 Sheets-Sheet 2 W. SCHLATTER El' AL HYDRAULIC OVERLOAD SAFETY DEVICE d mw...

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June 22, 1965 Filed June 11 1962 3,190,141 HYDRAULIC OVERLOAD SAFETY DEVICE Walter Schlatter, Schaan, Leichtenstein, and Max Fischer, Buchs, Sankt Gallen, Switzerland, assignors to Massey Schlatter A.G., Buchs, Sankt Gallen, Switzerland Filed June 11, 1962, Ser. No. 201,565

by means of an oil pump (not shown) similar to the pump in FIGURE 3,r if desired with anl accumulator interposed in order to obviate a permanent running of theoil pump. The oil pump has merely to cover the leakage losses and to make sure that in the cylinder space 12 a certain pressure is maintained, the so-called"pre'loading Claims priority, application Switzerland, June 13, 1%1, Y

6,881/61; Feb. 2, 1962,1,273/62 i 24 Claims.V (Cl. 745S4) The present invention relates to a hydraulic overload safety device comprising in combination: two relatively movable components, a hydraulic cylinder connected to one of said components, `a plunger slidable in said cylinder in the direction of the relative movement of said two components, said plunger and said movable components dening a chamber therebetween, a hydraulic fluid in operation lling the chamber bounded by the end of said plunger and said cylinder, said cylinder havingA a wall with a central aperture and said plunger having a neck portion passing with ample clearance through said aper- `ture and a valve head on said neck portion beyond said aperture normally contacting an inner or outer end face of said cylinder Wall adjacent said aperture forming a valve seat for said valve head, and an external source of hydraulic pressure in communication with said cylinder chamber, and in operation setting up a preloading pressure therein which keeps said valve head tightly on said valve seat as long as the pressure set up by theforce between said two relatively movable components is below said preloading pressure, and allows said valve head to be from overloading. The two components movable relative to one another may be in principle of any kind desired, and the overload safety device need not be structurally combined with any of them.

In the accompanyingdrawings, two embodiments of the invention are diagrammatically illustrated by way of example, in which: Y

FlG. 1 is a longitudinal section of arhydraulic safety device arranged on the carriage of a mechanical press, in the normal position; v

Cid

pressure. This pre-loading pressure may amount e.g. to

about 800 atmospheres gauge, but this ligureV should merely illustrate the order of magnitude of the pressures lin question. The pump is of each character that it precludes back-flow of uid. By the pre-loading pressure the spindle 5 is pulled downwardly by the plunger 3, so that the head 8 of the spindle contacts firmly and tightly the outer face 9 of the cylinder end wall 6, and accordingly no oil can emerge from the space 12 through the annular gap 14, which exists between the cylinder bottom 6 and the spindle 5, which in this region does not carry a screw thread. Y

The head 8 of the spindle is arranged within a cap 15, which is connected with the carriage 1 and the cylinder 2 to a rigid unit in a manner not illustrated in detail. 'Ihe head 8 of the spindle is provided on top with an internal toothing 16, which meshes with a pinion 17, mounted on a shaft 188 which is journalled in the cap 15 and passes through the same, and which carries another pinion 19 externally of the cap 15, Vwhich may be driven in a manner not illustrated in detail'in order toturn theY spindle V5 and thereby to adjust the plunger 3 with respect to the cylinder 2. The Vadjustment stroke of the plunger 3 is In the lower part of the cavity 22 of the cap 15 there is contained a small oil reserve 23, on top of whicha slight air excess pressure, of for example 2 atmospheres gauge,

is maintained by means of a compressed air pipe V24.

To the lower end face`21 of the plunger 3a tool holder Y (not shown) for a pressing tool is attached. The, op-V posite pressing tool is obviously arranged on4 the stationary tool support, towards which the carriage 1 forming the movable tool support moves during its working stroke. The cavity 25 existing in the lower portion ofthe hollow plunger 3 and bounded on top by the spindle 5 is incommunication with the cylinder space 12 througha duct 26 provided in the plunger 3, and Yis accordingly likewise lilled" with oil under pressure.

FIG. 2 is a corresponding section, but after the response of the safety device, and

FIG. 3 is a longitudinal section of a modified safety device. Y Y

` In FIG. l the `partly illustrated carriage of a mechanical press is denoted. The carriage 1 is moved up and down by means of an eccentric mechanism with respect -to a stationary tool support; these components of the press are not illustrated, since theyare well known. In the carriage 1 a cylinder 2 is lxed, in the interior of which a hollow plunger 3 is slidable, which has aniinternal screw thread 4 and is screwed on an adjustmentspindle 5. The spindle .5 passes through anopening 7 providedin the end wall 6 of the cylinder, and has a headr8, which normally contacts the outer face 9 of the cylinder-end wall 6. The rear annular end face 10 of the plunger 3, the spindle 5, the cylinder end wall 6 and the cylinder wall 11 delinea cylinder space 12, which is lled with oil. The oil is supplied to the cylinder space 12 through an oil pipe 13 The device described operates; as follows: When during the working strokev of the carriage 1 the force arising between the tools and the Work piece to be pressed does not exceed a predetermined limit, which is proportional to the pre-loading pressure, the plunger 3 moves as a rigid unit together with the carriage 1 and the components connected therewith. The tensile` force on the spindle genf erated by the pre-loadingV pressureabove the plunger v3V is reduced by the amount of the pressing force but yet holds the head 8 of the spindle tightly against the outer face V9 of the cylinder end wall 6.l As soon ,as the pressingV Y force exceeds the limit, the head 8 of the spindle is slightly lifted off the outer face 9 of the cylinder end wall,

so that oil from the cylinder chamberV 12 enters into the v cavity 22 of the cap through the gap 14. As soon as even alittle oil emerges from the cylinder space 12, the pressure in Vthis space12 and accordingly also the resistance Patented June 22, 1965 arsenal l i Y after. the response of the safety device to an excessive, pressing force. Y r l In the known hydraulic overload safety devices kthere v islikewisea plunger slidablein a'cylinderg-and a pre-loading pressure is maintainedin the cylinder chamber. To this cylinder space an outletduct Vis-connected, wherein` there isaV valve responsive to excess pressure. This;valve,`

' Vthe plunger 3 into the normal position after a response. ofV

Y the cylinder chamber 12 has to be switched off.

the Avalve head-*of Which is'loaded,-e.g., by means of a spring,.does not howeverwork so abruptly as thevalve 8, 9, sinceits valve head is subjectrto the bias ofthe spring Vforcing it towards .itsfclo'sed .position even. after the response pressurehas. been reached. Other` known valves likewisedo -not operateouicklyenough ,for the ,present purpose. It is accordingly very diicultwith-the'known devices tol pass duringavery short.period.of,.e.g., /o, of

afsecond sufcient oilfrorn the cylinder, space/throughV the Valve for preventing a'further rise of. the pressing force .duringl vthe Working strokeV andV thus to obviate` damage ,to theV pressing toolsand/or the press itself.. v

.The safety device described '.hasnot onlytheadvantage n ,that it respondsfvery rapidly and is accordinglyV applicable to mechanical presses having relatively highfnumbers of strokes `per minute-'and fior. which the known safety cle-- Vvices would fail, but has'the further: advantage that its Vconstruction solves. in .an extremely lsimply way even other problems of .importance `for arnechanical press.v

With the. known pressesvitis .usualto make, the toolY the device, for which purpose obviously the oil pressure in It is clear that .the safety Vdevice described could be tted alternativelytol the stationary tool support (not shown) instead of to' the movable Ytool'support 1. In

practice however, in viewY of the general assembly of the.

press and ofL its operation, the solution illustrated vwill be preferred., 10

, By the safety device illustrated in FIG. 3 'a considera.

able improvement in respectV of the embodiment accord#` ing to FIGSflV and 2 israttained therebyv that, instead ofy an outer face, an inner face otY a cylinder .wall is used as4 a valve seat, the headfof theplungerthen beingobvionsly f inside instead .ofroutside the cylinder.V

Y The following is a description of the'invention shown in -1 e ...Thecarriage (shown onlyy artially) of a mechanical press is `,likewise denoted .by thereference character 1,:

Vwhich-is'rnovable in a know-n manner-upanddown with;V

respect to a .stationary lower. tool support. (not shown).1.`

' The, carriage hasV avertical bore Stia, wherein a cylinder:

holder adjustable ,with respect to Vthe carriageiby means of i aspindle, in order .to .be capable of varying the distance;

between theitwo press tools. .'-InV order'to makethe play,V

inthe screw .thread of the spindle ineiective,it is necessary to arrange a lock nutat theupp'erv end thereof., vThis lock nut, which isY in a rather. inaccessible position at the` upper portion of the carriage, has .to `beloosened for any alteration in thepo'sition ofthe upper'pressing tool, and

With the device describedthislock-nutcan dispensed with, since the pressure oil takes'over its function thereby 'A that it holds the. plunger 3 in the `,lowest position ofthe play in the screw thread 'with respect to theA spindle 5;' For has to Ybe-iirmly tightened after .the adjustment of the spindle. I Y Y e" adjusting the plunger. 3, .itis only necessary. to :bring `the When the pressure is released for the adjustment of 'the plunger, ,the pressing of the. ,plunger against 4the cylinderf wall ceases so thatitdoes not hamper jthe Vadjustment of the,V plunger.

. It shouldbe remarked .that the duct n2.6 is-not indispensible', since`the play Vinithefthreadfof thespindle 5V may'.

constitute a sufficient Ycommunication-between the` chaml-V Vmounted a nut 39a,y which has a toothingtlq@ 1 The nut i 39 bears on an inner ange 41a of the carriage 1, on which Y. y a cap .15d is attached in la manner not shown in detail.'l The toothing 46a of the nut 39a' meshes ywith-alpini'on 17a, Vwhich, is mounted on :a shaft ljpassingthrough the cap thereby to adjust the plunger 3a together with the cylinderj` "n 2a in the vertical direction for the purpose of adjusting the n level of the lowerend face 21a,.of1 tlie cylinder-to, the` level bers 12 and 25. However, the duct 26 ,facilitatestheepasf sage of the oilfrom one chamberlto'the other, whenthe spindle iis` 'drivenby means of theV pinion 19 VV for the 1 adjustment ofthe plunger-3. Moreover thev duct'2r6 has` the eiect that Yupon responseof. the .safetykdevice the pressuref drops very rapidly also inthe .interior 25 .of the plunger, andfaccordinglygalso the. friction .between thef i `plunger and "the fcylinderlwalliis reducedgimmediately The ductf26 may Vnaturally beprovidedalternatively in th'espind1e5.if,` l. Y

The oil pressure prevailing inthe interior 25` of the plunger eiects a stiffening ,of VtheV bottom of the plunger z Vduring the operation lof `the press. They air pressure prep vailing on top of vthe oil reserve 23 servesV for restoring;

Y 2a is guided; Aplung'enjwhich is slidable inthe cylinder` 2a, is provided .with V,a head 8a serving-as a valve head,

which is however located insidesthe cylinder 2a instead off,

- outsiderthesame.: in the cylinder 2a abushing 6a having .i

an external screw thread .is'screweiwhich constitutesa. 'cylinder wall having'an aperture 7a;V Between the aper-L4 ture 7 andthe necklofthe plunger 3a there exists an an- .nnlar gap 14a. Theinnerface 9a of thecylinder wall which [serves as a .valve seat, gis. conical, and normally a complementary conical face 32a ofthe head Sa rests rmly and tightly on the same.

lncontrast to the FIGS. 1 and 2, .the cylinder Za is not j openon Vits end oppositeits wall 6a, jbutis. closed -by-a Y Y fulllower Vend -WallSwL and at its lowerend face 21a a toolk holder (not shown) for1.apress 'tool' is mounted.. The,` lother press tool is naturally mountedzon the stationarytool supports;towardsgwhich the carriage-1, which constitutes .i the movable,V tool support, moves during i-tsworking stroke; The cylinder space. .12a is lledwitheoil, whichk is supf plied toit throughfa central :bore-34o of the` plunger. 3d.

The bore 34a is :fed through a pressure oil pipe-13u11from,V

a high pressure pump 35a for.e.g.1l000 atmospheres gauge:

the, suction pipe 36a rif-WhichisA connected to anoil'reser-l I voir'ia. [The pump 35o is of such a characterthat it pre-VK cludes back-flow of f fluid.. The oil reservoir 23a is bounded by the .bushing 6a, .-a conical end Vface 37a ofthe'l cylinderY Zarand a, skirt Sprojectinggupwardly ,from-.the f same. v

UThe, plunger I3oz has ,an ,upper .portion .Soon which is 15a and carrying another pinion'lfal externally of the capL 15a, which may be drivenfin order :to turn `the nut 39a and of the 'press tools;k I f Adjacent 1 the lower end face Qof; the4 head 4.rzy of, the

'plunger the central bore 3410i the plungera has-1an f i annular shoulder Sfoon which bears the V.upper end of 7 av comparatively weakY spring 4211,'.Which'abutsbelow thei fbottorn vof the cylindersA The head iizLof` the plunger p has a lcylindrical'gmain por-tionASa, fthe diameter of which Y issmallerthan the inner, diameter of the'cylinder .Zalf

` This main'portionis followed, on-'top bya eonicallyV con:V

y'vergent portion with the conicalfsealingface 32o, and be'-l low `by, a portion. 44a converging oonioally downwardly.

.The device describe-doperatesasrfollows:

When during the .working strokel theforce arisingbe-.f

tween Ythe tools'and the -vvorrlrv pieceto bepressed doesnot exceed .a pre-determined pre-loadingpressure of e.g. 1000 atmospheres gauge prevailing in the cylinder space 12, .the cylinder 2 with the carriage 1 moves downwardly, the pressing force being transmitted to the cylinder 2a from the carriage 1 through the caplSa, the nut 39a, the plunger 3a and the pressure oil contained in the cylinder space 12a. As soon as the pressing force exceeds the limit, the cylinder 2a slides slightly upward with respect to the piston 3a, so .that the valve 8a, 9a opens, and oil flows from the cylinder chamber 12a, through the annular lgap 14a into the oil reservoir 23a. lConsequently the pressure drops in the cylinder chamber 112, so that the cylinder 2a moves abruptly upwardly into the position 2' indicated in chain-dotted lines, almost the whole of the oil contained .in the cylinder chamber 12a flowing extremely rapidly into the oil reservoir 23a.

When the pressure on the end face 21a of the cylinder 2a diminishes, the spring 42abiases the cylinder 2a away from the head 8a of the plunger, so that the oil flows back through the open valve 8a, 9a into the cylinder chamber 12a, and after the closing of the valve 8a, 9a, the Vhighpressure pump `35a restores the pre-loading pressure desired.

In order that the pump 35a need not operate continuously, an accumulator may be provided, which may for example be constructed as a pressure multiplier; As compared with the embodiment according to FIGS. 1 and 2, the present embodiment has the advantage, that no seal has to be providedin order to prevent a leakage of the pressure 4oil from the cylinder chamberllZa. b odiment according -to the FIGS. 1 and 2, the plunger has In the em-V a sliding surface guided on the inner wall surface of the cylinder, and along this sliding surface leakage of the pressure oil from the cylinder would be possible, or such a leakage has rto be preventedV by special measures, e.g. by providing piston rings, lwhich owing Ato the hi-gh oil pressure is a delicate task.

A further advantage of the present device is that for `the same limit of the pressing force, the cylinder 2a can be kept smaller than that of the device according to the FIGS. 1 and 2.

It is clear that the safety device according to FIG. 3 is likewise applicable not only to presses but also to other Y machines, e.g. rolling mills.

In FIG. 3 a modification is indicated, according toV which the oil reservoir 23a is closed on top by an Vannular cover plate 44a indicated in chain-dotted lines, which is sealed against the plunger 3a by means of a seal (not shown) `for example inserted into an annular groove, and by a further seal (likewise not shown) against the internal flange 41a. A pipe 45a indicated by a .single chain-dotted line issues immediately below the cover plate 44 into the oil reservoir 23a, which includes in this case moreover a small space existing above the skirt 38a in the interior of the carriage 1. The pipe 45a may be supplied from a source 46a with a fluid, for example air or oil under a comparatively low pressure, of i.e. 5-10 atmospheres gauge.- Thereby the necessity is obviated of providing the spring 42a, whichas explained herein aboveeliects a closure of the valve 8a,9r after a response of the device to an overload, oil flowing from the reservoir into the cylinder chamber 12a. The return ow of the oil and the closure of the valve is here effected by the compressed air or by the oil supplied under low pressure e.g. by an oil pump having an output which is large as compared with that of the high-pressure pump a. After the closure of the valve 8a, 9a, Vthe high-pressure pump 35d restores the required high pressure of e.g.'V

1000 atmospheres gauge in the cylinder Vchamber 12a.

which there is contained partly oil and partly air of e.g. 5-10 atmospheres gauge, the low-.pressure oil pump feeding oil continuously into the container until the prescribed p pressure is attained therein, whereafter the pump is switched off or is made to work idly over a by-pass valve.

If desired, an observation glass may be provided for indicating the oil `level in the oil reservoir 23a.

While we have herein described and illustrated in the accompanying drawing what may be considered typical and particularly useful embodiments of our said invention we wish it to be understood that we do not wish to vlimit ourselves to the particular details and dimensions described and illustrated; for obvious modilications will occur to a person skilled in the art.

What we claim as our invention and desire to secure by Letters Patent is:

1. A hydraulic overload safety device comprising in combination: two relatively movable cylinder components, a hydraulic chamber in one of said components, a plunger carried by one of said components and movable in saidr chamber in the direction of the relative movement of said two components, said chamber .being delined by said plunger and said movable components,` a hydraulic pressure uid from an external source of hydraulic pressure in operation iilling the space' in said chamber, vsaid chamber having a wall provided `with a. central aperture and said plunger having a neck portion passing with ample clearancethrough said aperture, a valve head on said neck portion beyond said aperture, said central aperture in said wall forming a valve seat Ifor .said valve head, means lfor subjecting said movable cylinder components to external forces, and an external source of hydraulic pressure in communication with said chamber and in operation setting up a preloading pressure therein which keeps said-valve head tightly on said valve seat as long as the pressure set up by the external forces between said relatively movable cylinder components is below said preloading pressure, and allows said valve head Vto be lifted olf said valve. seat and Yto rapidly discharge said hydraulic uid in said chamber past said neck portion when external forces generates a pressure exceeding a predetermined pressure limit of said preloading pressure.

2. A`device as claimed in claim 1, wherein said component, with said hydraulic chamber, is a movable toolV support, andthe other said component is a iixed tool support.

3. A device as claimed in claim 1, wherein one of said componentshas a hollow portion with an internal screw thread, and comprising a threaded plunger screwed intoVV said internal screw thread and forming said neck portion carrying said Vvalve head, said valve head contacting an outer face of said cylinder wall serving as said valve seat.V

4. A device as claimed in claim 3, wherein said chamber contains a duct connecting the space between said chamber and the end of said threaded plunger with the space adjacent said apertured cylinderwall.

5. A device as claimed in'claim 3, comprising a cap iixedly connected to a cylinder forming said chamber outside said apertured chamber wall, said valve head being located between said cap and said chamber wall, and the space between said cap and said chamber wall containing pressure loaded hydraulic iiuid.V Y

' l6. A device as claimed in claim 5, comprising an external source of compressed air in communication with said space between said cap and said cylinder wall.

7. A device as claimed in claim'S, comprising a toothed gearing in driving connection with said threaded plunger, and capable of adjusting the axial position of said plunger relativeV to said chamber by turning said threaded spindle in said internal screw thread of said plunger.

8. A device'as claimed in claim7, wherein said toothed V direction of relative movement of said two cor'nponent,Y

Vber bottom.

Vsaid second, full,rcylinder bottom. ,Y

` 13. A device as claimed in claim 9, wherein .said

Y Y ry g Y saidVchamberrbeingrdened by said movable components and saidplunger, a pressure loadedV hydraulic fluid Ifillingr the space in said Vchamber,,said Vchamber having ka firstz i9. Ai device as .claimed inA claim t18,`wherein said fluid reservoir is sealed outwardly, and comprising an ex- Wall with acentral Vaperture and said plunger having a neckportion passingA with ample clearance through said aperture, a yvalve head on said neck portion Vbeyond saidV aperture, 'an inner `face of said chamber wallforming .a

valve seat for said .valve head, means forA subjecting said movablel components to externalttforces, and in externalv source of hydraulic pressure in communication with said Y chamber and in operation settinguup a preloadngpres- Y sure therein which keeps said valve Vhead tightly onsaid Y valve seat as long .as :the pressure `setup bythe external forces between said relatively movable. cylinder components is below said'preloa'ding pressure,.and allowsfsaid a/alve head tobe lifted ol said rvalve seat and to rapidly` Y discharge said hydraulic uid pastsaid neel; portion when said external forces generates a pressure further compressl ing the pressureload'ed hydraulic uid until said external -V forces exceed a predetermined pressure limit.

. 10. A device asclaimed in claim'9, ,inl which the cylin- 'der component` is provided withV an end wall having anV components and plunger`,:an enlarged head on said plungerV f outer gend face in operation serving Vfor the attachment Y `oan external tool'holder(V Y 1i. A device as claimed in claim 9, comprising a bush'-,

VVVingscrevved into said cylinder-component, an end `lface y of movable saidbushingwformingrsaid apertured chamlZ. A device, as

Y claimedk ,in claim 9,' vvliereinV said plunger has a longitudinal bore. incom'munication with, 9

said external source ofhydraulicpressure andwith theV cylinder space bounded'between saidvalvejhead andA K plunger hasA an externally screw-threadedV portion,and

comprising a nutrserewedkon said screw-threaded portion servingV forradjusting said plunger relative lto. said.

component to which is it attached.

14. A device as claimed in claim 9,comprisingw ac'omfV pression spring arranged between said'second, full, chai-ng.

ber bottom and said, valve head. Y

15. devicey as claimed in claim 9,-comprising a fluidV` reservoirarranged outside adjacent .said rst, apertured,4

chamber bottom.

16. A device as claimed inclaimV lfcomprising a duid level indicator in communication with said fluid reservoir.

Il?. A device as claimed in'claim 15, .comprising an external high-pressure uid pump having a suction pipe tween said valve head and said cylinder bottom. d. .A device as claimed inclaim 17,5 comprising-.a

. Skirt arranged on said cylinder and circumferentiallyV bounding said uid reservoir. Y

Y dipping'into saidV fluid reservoir, and a pressureV pipe Y communication with said chamber.'space bounded be 5G' Y source of lowerffluid. pressure Vsupplies air;Av 'Y' of fluidas said lluid contained in said hydraulic chamber. 22...A" devicefas claimedjnjclaim 21, whereinl said"` sourcer of.lower.luid pressure comprises 1an oil pump;

23. yA 'device asY claimed, in claim 2:1, Whereinsai-d.` source of klower lfluid pressure-, comprises an "oil .pressure1 accumulator.

Y 24. A hydraulic overload safety device comprising inner' and outer relatively movable! cylinder components, a 'restricted walllportion formedin' one of said components, aj plunger Vadjustably mounted in the other of said .cylin- Y der components projecting througlranY aperture in said.

wall portion, a chamber delinedby said movable cylinder movable insaid chamber on ,one side of said wall portion, said chamberbeing pressure'loaded with a hydraulic fluid illing'the space in said chamber, external pressure meansv for supplying 1saidhydraulic fluid vto said chamber, the

aperture in said wall portion being of a diameterslightlyf greater than `said plunger togprovide a clearance space.

through theV aperture in said Wall, means -forsubjecting said kmovable cylinder components to Vexternalforces,` Ysaid enlarged head and 4said wall surrounding said aperture lforming a .-valve ystructure inA a normally closed'posi-,Y` tion which isY opened when Vthe external forces between .said cylinder componentsfor further compressingthe` pressure'loaded hydraulic Huid until said external forces Vexceeds a predetermined pressure limit, whereby theY hy-V Y draulic liuid in said chamber will escape through the apert` ture in.said'wall and permit relative movement betweenr the plunger and movable `cylinder component. i

Referencesited Yby the Examiner o `UNVITED YSTATES PATENTS' BROUGHTON; G.y Erimaryl."xan'zinier.V 'i rsADon WELL, Examiner.` 'Y

said 

1. A HYDRAULIC OVERLOAD SAFETY DEVICE COMPRISING IN COMBINATION: TWO RELATIVELY MOVABLE CYLINDER COMPONENTS, A HYDRAULIC CHAMBER IN ONE OF SAID COMPONENTS, A PLUNGER CARRIED BY ONE OF SAID COMPONENTS AND MOVABLE IN SAID CHAMBER IN THE DIRECTION OF THE RELATIVE MOVEMENT OF SAID TWO COMPONENTS, SAID CHAMBER BEING DEFINED BY SAID PLUNGER AND SAID MOVABLE COMPONENTS, A HYDRAULIC PRESSURE FLUID FROM AN EXTERNAL SOURCE OF HYDRAULIC PRESSURE IN OPERATION FILLING THE SPACE IN SAID CHAMBER, SAID CHAMBER HAVING A WALL PROVIDED WITH A CENTRAL APERTURE AND SAID PLUNGER HAVING A NECK PORTION PASSING WITH AMPLE CLEARANCE THROUGH SAID APERTURE, A VALVE HEAD ON SAID NECK PORTION BEYOND SAID APERTURE, A VALVE HEAD ON SAID IN SAID WALL FORMING A VALVE SEAT FOR SAID VALVE HEAD, MEANS FOR SUBJECTING SAID MOVABLE CYLINDER COMPONENTS TO EXTERNAL FORCES, AND AN EXTERNAL SOURCE OF HYDRULIC PRESSURE IN COMMUNICATION WITH SAID CHAMBER AND IN OPERATION SETTING UP A PRELOADING PRESSURE THEREIN WHICH KEEPS SAID VALVE HEAD TIGHTLY ON SAID VALVE SEAT AS LONG AS THE PRESSURE SET UP BY THE EXTERNAL FORCES BETWEEN SAID RELATIVELY MOVABLE CYLINDER COMPONENTS IS BELOW SAID PRELOADING PRESSURE, AND ALLOWS SAID VALVE HEAD TO BE LIFTED OFF SAID VALVE SEAT AND TO RAPIDLY DISCHARGE SAID HYDRAULIC FLUID IN SAID CHAMBER PAST SAID NECK PORTION WHEN EXTERNAL FORCES GENERATES A PRESSURE EXCEEDING A PREDETERMINED PRESSURE LIMIT OF SAID PRELOADING PRESSURE. 